The incredible complexity of mammals, despite their relatively limited number of genes, stems in significant part from epigenetic regulatory mechanisms. Epigenetic changes are heritable, but readily reversible, changes in the expression/function of genomes that do not involve an alteration in DMA sequence information. DNA (de)methylation and histone modification are the two most common mechanisms of epigenetic change. Thus, epigenetics has emerged into a prominent field, as the biological roles of DNA methyltransferases (MTases) and histone-specific modifying enzymes [e.g., (de)acetylases,, (de)methylases, kinases and phosphatases] in chromatin remodeling and regulation of gene expression have been discovered. Our long-term goal is to understand how the initial methylation pattern (de novo methylation) is generated, i.e, mammalian DNA MTases select some CpG sequences for methylation while sparing others. This grant will dissect the structures and functions of the DnmtS family of DNA [cytosine-5] MTases and the methyl CpG binding protein MeCP2, which play important roles in genomic imprinting, chromatin (re)modeling and human diseases (ICF syndrome and Rett syndrome). Our specific aims are: 1. To dissect the domain structures and functions of the DnmtSa and DnmtSb MTases. In ICF patients, mutations have been found in the catalytic domain of DnmtSb. 2. To determine whether DnmtSL acts as an autogenous regulator by self-oligomerization 3. To test the hypothesis that DnmtSL stimulates DnmtSa activity by unmasking its catalytic domain 4. To test the hypothesis that Rett syndrome can result from mutations in a self-association domain of MeCP2